|
|
Criteria Air Pollutants
Learn about key air pollutants and their emissions inventory for the San Francisco Bay Area.
Regional Criteria Air Pollutants Inventory
Criteria air pollutants (CAPs) are a set of commonly-occurring pollutants regulated by the U.S. Environmental Protection Agency (EPA) under the Clean Air Act to protect human health and the environment. The CAPs include particulate matter with diameters 10 micrometers and smaller (PM10) and 2.5 micrometers and smaller (PM2.5), ground level ozone (O3), nitrogen dioxides (NO2), carbon monoxide (CO), sulfur dioxide (SO2), and lead (Pb). These pollutants serve as indicators of air quality. The EPA has established National Ambient Air Quality Standards that define acceptable concentration levels of each pollutant in ambient air.
To track emissions from these pollutants, the Air District maintains a comprehensive regional CAP Inventory that estimates emissions of CAPs and their precursors from human activities across the Bay Area. Emissions are organized by source categories and aggregated into sectors. The inventory demonstrates compliance with clean air laws and supports ongoing development of pollutant reduction strategies through air quality planning, modeling, and rulemaking.
Ground level ozone is different from other CAPs because it is not emitted directly into the atmosphere. Instead, O3 forms through chemical reactions between nitrogen oxides (NOx) and reactive organic gases (ROG) in the presence of sunlight. For this reason, the regional inventory tracks ozone precursor emissions of ROG, and total organic gases (TOG) in addition to NOx. Pb emissions from permitted sources are included in the Air District’s toxic air contaminants reports.
The current inventory uses a base year for backcasting and forecasting emissions from 1990 through 2040 across various categories. An overview of the Bay Area’s emissions inventory for CAPs and ozone precursors is provided in the Bay Area Emissions Inventory: Summary Report for Criteria Air Pollutants. The Air District periodically updates the inventory to incorporate the latest available data and to improve emission estimation methods. A new regional inventory with an updated base year is planned for release in 2027.
Detailed documentation describing the inventory methodologies, assumptions, and emission estimates are provided in the table below. The accompanying CAP Emissions Lookup Tool provides an interactive way to explore emissions by pollutant, sector, subsector, and year.
CAP Emissions Inventory Reports and Targeted Studies
The following table provides access to current and historical regional CAP emissions inventory reports including detailed descriptions of the methodologies used to estimate emissions by source category. Emissions from individual categories are aggregated across defined sectors and subsectors.
The Air District also conducts and sponsors targeted studies to better understand emissions sources, improve estimation methods, and evaluate potential emissions reduction measures. These studies are listed below, and they support continued improvements to the accuracy and overall quality of the regional CAP emissions inventory.
Last Updated: 10/14/2026